First of all this is a hit and run tactic using the fastest units in the game. Do it properly and you will gain the advantage early on.

Build order: builder 1 tunnel builder 2 Spider Pit Spiderlings builder 1 tunnel builder 2 tunnel Update pit lvl 2 Spider Riders if they build pikes/spears, either save for shelob/custom hero or build goblin cave and counter with goblins. (hold them up with goblins and use spider riders as archers, works a charm) Don't worry about upgrades at this point just keep pumping units, you will win!

As soon as the spider pit is up build spiderings and send them straight to the enemys base targeting resource buildings and builders, avoiding enemy troops and the fortress. If they engage you with their troops then run your spiderlings away go a long way round and attack from the other side. Keep them on squad 1 and make sure you complete the build order above at the same time as hit and runs with your spiderlings. Use attack stance and war chant just before you get to the first resource.

Once the above order is done and you've gained the upper hand, build spider riders and keep sending them at the base, once again staying away from the fortress and spearmen. If the enemy then starts building spearmen, use your spider riders in arrow mode. When you feel you have enough units to destroy the fortress go ahead and send them in!